The present invention relates to an electrical current transducer comprising a magnetic circuit with a magnetic core having an air-gap. The present invention is in particular related to an electrical current transducer with a wound magnetic core.
Many conventional current transducers comprise a magnetic core made of material with a high magnetic permeability and a magnetic field sensor, such as a Hall effect sensor, positioned in a gap formed in the magnetic core. A primary conductor extending through a central passage of the magnetic circuit generates a magnetic field that is picked-up by the magnetic core. The magnetic field flows across the gap and the magnetic field detector positioned therein. Since the gap represents a zone of low magnetic permeability and thus has an important effect on the magnetic field lines, it is important to accurately control the width of the gap in order to ensure accurate and reliable measurement of the electrical current to be measured.
It is also important to reduce losses in the transducer, in particular losses due to the formation of eddy currents in the magnetic core and to avoid magnetic saturation along any section of the magnetic core. The use of stacked laminated sheets to reduce eddy currents is well-known. A known means of forming a stacked multilayer magnetic circuit is by winding a thin band or strip of magnetic material to form an annular wound core. It is known to provide wound cores with air gaps, whereby the manufacturing process consists of first winding an annular toroidal core, subsequently applying resin to the core to hold the concentric layers of strip material and subsequently machining a gap radially through a section of the winding. Once the resin has been applied, annealing of the material of the wound magnetic core is difficult or no longer possible in view of the high temperatures required for the annealing process.
Working of materials with high magnetic permeabilities can affect their magnetic properties, in particular by increasing hysteresis effects and thus adversely affecting the magnetic performance of the magnetic circuit.
The gap length of a magnetic circuit may vary due to thermal and mechanical forces. It is known to stabilize the size of the gap by means of an element fixed to the magnetic core. In JP 2 601 297 the air gap of an annular wound magnetic core is fixed by means of a T-shaped element having a portion partially inserted in the air gap from the outer radial side of the magnetic circuit, the insert being held in place by means of a band wound around the magnetic circuit and the insert. A drawback of this design is that the insert partially engages in the air gap and thus limits the space for insertion of a magnetic field sensor. Moreover, the insert only engages the outer peripheral layers of the magnetic circuit and thus does not prevent variation of the size of the air gap of the inner radial layers of the magnetic circuit, in particular variations due to thermal forces that the resin binding the layers cannot entirely prevent. Also, heat treatment of the magnetic circuit after application of the resin is either not possible or at best limited. The position of the insert from the outer radial periphery of the magnetic circuit also increases the size of the magnetic circuit.
In EP2224461, a magnetic circuit with a wound core and a bridging element welded on a lateral side of the core and spanning across the air gap, is disclosed. This offers a compact configuration with stable fixing of the air gap. The welded connection between the bridging element and the magnetic core may however reduce the magnetic permeability of the magnetic core in the welding zone. Moreover, in particularly harsh environments, such as those found in automotive applications, the weld connections subjected to thermal and mechanical shock as well as a corrosive environment may deteriorate over time.
It is an object of this invention to provide an electrical current transducer having a magnetic circuit with gap, that is robust and economical to manufacture, especially in large series manufacturing, yet ensures accurate and reliable performance even when subjected to harsh operating environments.
It would be advantageous to provide an electrical current transducer having a magnetic core that has uniform magnetic material properties, in particular a high and uniform magnetic permeability.